Devices such as tactical radios often use an indicator signal to command the radio to begin transmission. The most common method of accomplishing this command is to provide a mechanical switch on the radio's handset. This switch provides an electrical signal to the radio to begin transmission. In other applications, this Push-to-Talk (PTT) signal is generated either electronically or electromechanically (via a relay).
The normal signaling protocol is for a closure to ground (zero volts) to command `transmit`. The command "don't transmit" is exemplified by an open circuit or via a high potential voltage or low voltage across the terminals of a circuit.
Due to the nature of how PTT signals are generated, with mechanical switches and electromechanical relays, there is often significant noise and high-voltage ringing present on PTT lines. In some applications, significant Lengths of cable are present on the PTT line providing additional opportunity for high voltage transients and noise to infiltrate the PTT interface.
Previous designs of PTT interfaces have used circuits with high cost components or large numbers of components. FIG. 3 shows the traditional design used in the present production of a push-to-talk SINCGARS radio. As shown in FIG. 3, such design requires 2 transistors (Q.sub.1,Q.sub.2)and 13 total components including a power supply. FIG. 4 shows another prior art implementation of a PTT interface. This design, while requiring only nine components, however, further includes a costly differential amplifier 100 and also requires both a positive supply V2 and negative supply voltage V3 to operate. The high component count, high cost, and the need for negative supply voltages associated with prior art PTT interface circuits make it extremely desirable to obtain a low cost, low power interface circuit which is operable with a minimal number of components.
As can be seen in FIG. 1, the design according to the present invention overcomes these shortcomings by the judicious use of a single, low cost field effect transistor. Current routing with diodes precludes the need for a negative supply voltage.